Verifiability - show vote hash after casting a vote

[chenchanglew]

Implementation step of Verifiability Documentation

1. frontend: show a user the hash of their encrypted ballot. (This issue)
2. smart-contract + proxy: add users' hash of encrypted ballot (sha256) in the form. The hash will send along with the getForm to the user. (Issue: Verifiability - add users' hash of the encrypted ballot for each election. #240)
3. frontend: In the election info page, show a list of the voter who vote and their hash of encrypted votes got from the proxy. (Issue: Verifiability - show hash of encrypted vote in election detail pages #241)

How:
After a user cast a vote and send it to the backend, the frontend will pop up a message (like how you show succeed message after creating an election) showing the hash value (sha256) of the encrypted ballot to the user.
The user can later use that hash value to check if his/her ballot is cast correctly in the blockchain.

Where:
web/frontend/src/

Target:
Users can able to see the hash of the vote.
The hash will only show once.
Users can click a copy icon button to copy the hash of the vote. (optional)